HOW A TURBOCHARGER WORKS

Basic functioning of a four-stroke diesel engine

Most of you know how a diesel engine works, but for the ones without this knowledge, we mention a brief description of the basic principles, to better understand the function of the turbo as a part of the total engine concept.

            

 A diesel engine is a so called self igniter.

Air is compressed under high pressure within the cylinder, where also fuel gets injected, which self ignites caused by the high compression temperature and pushes the piston downwards into the cylinder.

This downwards force is transferred to an out of centre tap on the crankshaft by means of a pushrod and in this way changed into a rotational force.

The major difference with a gasoline engine is, that the diesel/air mixture self ignites, so there is no need for a separate ignition system.

However, there always needs to be an injection system, a high pressure fuel injection pump and nozzles, or as with more modern engines with direct injection, under high pressure through injector pumps per cylinder.

If air is compressed to high pressures, it is heavily heating up.

Even to such high temperatures, that fuel injected at high pressure is self igniting spontaneously.

A four-stroke engine is characterized by four strokes, 4 following strokes of a piston in a cylinder with different functions.

                           

  1)  intake-stroke    2) compression-stroke    3) power-stroke  4) exhaust-stroke

The intake-stroke (1)

(piston ↓) with a diesel engine only air will be forced into the cylinder through the opened intake valve.

The compression-stroke (2)

(piston ↑) the air is compressed by the upward movement. Caused by the high compression ratio of a diesel engine the compressed air heats up sufficiently  (700 à 900°C) to self ignite the diesel fuel. Both intake- and exhaust valves are closed.

The self ignition temperature of diesel fuel is 320 à 360°C. At the end of the compression stroke the fuel is injected. The ignition produces a heat expansion and pressure of 60 à 180 bar at a temperature of 2000 à 2500 °C.

The power-stroke (3)

(piston ↓) caused by the high ignition pressure the piston is driven downward.

The exhaust-stroke (4)  

(piston ↑) the hot exhaust gasses escape through the now open exhaust valve, supported by the upward moving piston.

The first 2 strokes occur during 1 revolution of the crankshaft. During the following revolution the next 2 strokes are made.

Because each up- or downward movement of the piston is called a stroke, this engine is called a four-stroke engine.

The flow of hot exhaust gasses, which leave the engine through the exhaust manifold towards the exhaust system at a normally aspirated engine, is now used to drive a turbine (of the turbocharger) , which is fitted on and between the exhaust manifold and the exhaust system.

How works an engine

How works a turbo

 

HOW A TURBO WORKS

      Schematic turbocharged diesel engine system

 

1 Compressor Inlet   2  Compressor Discharge            3 Charge air cooler             
4 Intake Valve           5 Exhaust Valve                           6  Turbine inlet                      
7 Turbine outlet

The components that make up a typical turbocharged diesel engine system are:

1)
The air inlet where ambient air is entering the compressor of the turbo.

2)
The air is then compressed to increased density (mass/unit volume flow). Air enters the compressor at a temperature equivalent to atmosphere, but as compression causes the temperature to rise, it leaves the compressor at temperatures of 200 degrees C and more at high boost applications.

3)
Some tractorpulling classes are allowed to have a charge air cooler (intercooler) that cools the compressed air to further increase its density and thus the amount of oxygen in same volume of air charge.

4)
After passing through the intake manifold, the air enters the engine’s cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given engine size or displacement.

5)
After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke into the exhaust manifold.

6)
The high temperature gas then continues on to the turbine. The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure.

7)
A pressure and temperature drop occurs (heat expansion) across the turbine, which harnesses the exhaust gas energy to provide the power necessary to drive the compressor.

 

A TURBOCHARGER CONSISTS OF 3 MAIN SECTIONS

Turbine section                               Turbine housing and Shaft + Turbine wheel

Compressor section                       Compressor wheel (Impeller) and Compressor       housing

Central section                               Bearing housing and bearing system

                                              

                                    Turbocharger exploded view

           

TURBINE SECTION

The turbine section consists of 2 major components

Turbine housing                              The turbine housing is bolted to the exhaust manifold  of the engine. The exhaust gasses are used to rotate the turbine wheel, which has been positioned in the turbine housing. Turbine temperatures up to 1100 degrees C in Truck- and Tractorpulling applications

Turbine wheel + shaft assy          The turbine wheel is friction welded to a forged steel shaft, which in turn rotates the compressor wheel, which is fitted at the opposite shaft end and secured by a locknut

 

The heat energy, pressure and flow from the exhaust gasses are driving the turbine wheel, when these are guided through the turbine housing. The hot exhaust gasses are passing an opening of a defined size in the turbine housing (nozzle area).

After having passed the nozzle area, the hot exhaust gasses expand and kinetic energy from pressure drop, flow and heat drop/expansion drive the turbine wheel to high speeds. Once the speed of the engine increases and more exhaust gasses pass the nozzle area of the turbine housing, the turbine wheel assembly and compressor fixed to it, will increase in speed. The result is increased air pressure and air flow charge (mass flow) to the engine cylinders. (blue arrow indicates position of nozzle area).

Turbine shaft + wheel assembly

COMPRESSOR SECTION

The compressor section consists of 3 major components

Compressor wheel (Impeller)      As the compressor wheel spins, air is drawn into the compressor from the atmosphere, the air will be compressed and forced under pressure into the engine cylinder

Compressor housing                      The compressor wheel is positioned in the compressor housing with tight and strict tolerances for the clearance between housing wall- and wheel contours, to achieve the highest possible compressor efficiency

                                 

                               Pictures of some components

                                         

                   Compressor housing                  Impeller wheel

Diffuser   The compressor section is designed to convert and slow down an air stream of low pressure and high speed and turbulence, into as a stable air stream of high pressure and low speed. This is achieved by a Diffuser system, a defined opening between de compressor housing wall and the wall of the volute backplate area at the bearing

Air enters the compressor at ambient temperature. The process of compression causes the air to increase in temperature up and over 200 degrees C, depending on the turbo pressure generated on a particular application.

                 

CENTRAL SECTION

The central housing (bearing housing) contains the bearing system, which consists of 2 free floating rotational journal bearings (radial) and 1 thrust bearing (axial). The journal bearings are made from leaded bronze or copper/brass. The thrust bearing is made from phosphor bronze or sintered iron. All bearings are lubricated and cooled by the engine oil.

Typical oil film thickness is 0.008 to 0.015 mm, the thickness of a human hair. The bearing material has to withstand high temperatures, generated in the turbine- and compressor section as well, which is transferred to the central housing and the bearings. On top of this the bearing system has to coop with hot engine shutdowns, soot in the lubrication oil, contaminants, oil additives and dry starts.

         

BEARING SYSTEM AND LUBRICATION       

The turbo bearing section is an integral part of the engine lubrication system, so the lubrication oil from the engine oil pump is fed under pressure into the bearing housing, towards the journal bearings and the thrust bearing system.

The bearing system used is called a hydro-dynamic bearing system. 2 rotating journal bearings rotate on the shaft and inside the bores in the bearing housing, to control the radial shaft movements and vibrations.

To control the axial rotor movements, a  tapered land type thrust bearing has been positioned at the compressor end of the bearing housing. The thrust bearing is fixed and does not rotate. The lubrication system of the thrust bearing has been arranged with a steel thrust collar supporting the thrust bearing at the bottom side. At the top side with the bottom side of the oil slinger. Because of the tight tolerances and heavy axial load on the rotor, both parts have specially hardened and lapped mating faces towards the thrust bearing.

Again, the entire lubrication system works with a very thin film of oil. The result is, that the complete rotor is ‘free floating’ on a thin film of oil, both axial and radial.

Bearing system and lubrication 

                         

    Pictures of some components

         

  Journal bearings       Thrust bearing           Thrust collar             Oil slinger

                                   (Center) Bearing housing       Heatshield

OIL SEALING SYSTEM – PISTON RING SEALS

At both ends of the rotor shaft, piston ring type seals have been fitted. At the turbine side the piston ring(s) is/are positioned in a groove on the shaft hub, behind the turbine wheel. The piston rings are seated in the bore of the bearing housing. At the compressor side the piston ring is positioned in a groove on the oil slinger bush and seated in the bore of an oil seal plate behind the compressor wheel.

The piston rings are no typical oil seals. The main purpose and function is to prevent ventilation of both exhaust- and boost pressure entering the bearing housing. The engine lubrication oil reaches the (rotating) journal bearings under pressure, causing the oil to mix up with air, resulting in a ‘creamy substance’. After having passed the bearings, the oil therefore has to return to its normal viscosity in the carter of the bearing housing and then flow freely down, without any restriction, to the engine sump through the oil drain pipe.

The piston ring arrangements therefore only partly support oil sealing, the oil sealing is mainly effected by the positive pressure in both the turbine and compressor section.

 

                              (Note the above enlarged piston ring sections  in circles)

        Pictures of some components

            

                  Piston rings                Oil slinger                  Oil seal plate